1. Field of the Invention
The present invention is related to a method for manufacturing a golf ball and is particularly concerned with an improvement of compression molding of a golf ball or a sphere that forms a part of a golf ball.
2. Description of the Related Art
Solid golf balls having various types of layer construction are commercially available. For example, there are one-piece solid golf balls having a single layer, two-piece solid golf balls having a cover and a core of a single layer construction, and multi piece solid golf balls having a cover and a core of two layers. The two layers core comprises a center and a mid layer covering the center. One-piece golf balls and the cores of two-piece golf balls and the center and the core of the multi piece golf balls are all spherical. These spheres are generally made of a rubber composition.
A mold for manufacturing golf balls comprises an upper portion and a lower portion. Both upper and lower portions have a hemispheric cavity respectively. When the upper portion mates with the lower portion, a spherical cavity is formed by a pair of hemispheric cavities. When a core for two-piece golf balls is molded, a preforming material (which is referred to as a “plug”) is placed in the mold. The preforming material is made of a rubber composition. Volume of a preforming material to be placed in each cavity is larger than that of the core to be molded. In other words, the volume of the preforming material is larger than the capacity of the spherical cavity. Due to this difference, when the mold is clamped gradually, excessive rubber composition outflows from the cavity. In accordance with the outflow of the excessive rubber composition, air is also released from the cavity. The mold is clamped and followed by heating to a predetermined temperature, and is kept at the temperature for a predetermined period of time. Thereby, a crosslinking reaction of a rubber initiates, thereby the rubber composition is cured to form a core. Thereafter, the mold is opened and a core is picked up from the mold.
Residual air in the cavity during the clamping step, if any, may generate convex portions (which is referred to as “bare”) or tinned portions (which is referred to as “scorching”) on the surface of a molded core. Particularly, when a preforming material of a cylindrical shape is placed in the cavity, a closed space is easily defined between the end portion of the preforming material and the interior surface of the cavity. The air remaining in such space is released form the cavity with difficulty.
One of the means to release the residual air from the mold is to use a large volume of preforming material so that a large amount of rubber composition outflows from the cavity with the residual air during the clamping step. This means is not sufficient to prevent a production of the above-mentioned defective cores, even if it may decrease the production of defective cores in some extents. Further, as the outflowed rubber composition is hardly recycled and needs to be disposed, a yield of the rubber composition becomes low, which results in cost increase of golf ball manufacturing.
Similar problems arise with a compression molding for manufacturing a center and a core of multi piece golf balls and one-piece golf balls.